Over the course of the previous 20 years, Scottish Soay sheep have been studied on the island of Hirta in the Outer Hebrides. This study revealed that weather patterns were driving changes in body shape and population size. Harsh winters led to larger sheep, which brought about changes in population size, but these effects were not seen in milder winters.
"Until now, it has proven really quite difficult to show how ecology and evolutionary change are linked, but we have developed a way to tie them together," said Tim Coulson, an author of the Science paper and a scientist at Imperial College London.
"The reason we looked at these sheep is they have been studied in enormous detail. Where they live is like a natural laboratory - it is a really simple system -- there is just sheep and grass on the island," Coulson explained.
The scientists collected and analyzed data about sheep population sizes and body size since 1985.
"To determine how ecology influences evolution and vice versa, an important step is to be able to see how population dynamics are influenced by traits such as body size or eye colour that are, in part, controlled by genes."
The research team found that body size was linked to the numbers of animals present: when the sheep had large body sizes, population numbers fluctuated widely, possibly because body size is linked to reproductive success. The study also showed that larger sheep were genetically favored during the harsher winters during the 1980s.
"But over the years, winters have been getting a little bit better; and as winters have got better, we have found there is not as much natural selection for large animals as we saw in the past, as there is less advantage to being big."
The study revealed how environmental factors were driving evolutionary and ecological change, and predicted that, as the climate changed and winters became less frequently harsh, the sheep would get smaller and the population size would become more stable.
"People have argued for a long time that climate change is leaving an ecological legacy, but we have shown it will leave an evolutionary legacy too," Coulson added.
Cited story.
Comments
I attended a lecture on the Souay Sheep by one of the researchers. The data is really quite spectacular, as they have individual case histories for every single sheep born on the island.
Posted by: John Wilkins | March 17, 2007 10:35 PM
This is one of the areas I work in (but not, alas, on this data). There are now a few good quality data sets like this, mainly on birds and mainly from Europe (at least the ones that I'm aware of). Some of these data sets go back longer, even if they are not as complete.
With my statistician's hat on (it's bell-shaped), I think this is great: the analyses need some quite complex statistics, and some big computers. But there are some developments that can be made, and I'm hoping this will happen, because then we can really get into the details of what is happening to these populations. Exciting times ahead!
Bob
Posted by: Bob O'H | March 18, 2007 5:11 AM
This brings back memories. I volunteered to go to St.Kilda (Hirta is the main island) in 1996 and spent six weeks observing and recording the mating behaviour of Soay sheep, all in the most appalling weather conditions. And that was one of the mild winters. It is good to see the findings coming out of this long term study and nice to think that I contributed a little.
Posted by: dc | March 18, 2007 11:52 AM